This invention relates to a conrol system of an alternating current motor, and more particularly to a system of controlling the speed of an AC motor by using a frequency converter including serially connected rectifier and inverter.
As shown in FIG. 1, an induction motor 15 is energized by an AC power source 11 through a rectifier 12, a DC reactor 13 and an inverter 14. The rectifier and inverter comprise semiconductor switching elements such as thyristors or power transistors.
One example of the prior art control system is constructed as shown in FIG. 1 in which the outputs of a speed reference setter in the form of a variable resistor 21 and of a speed detector in the form of a tachometer generator 16 coupled to an induction motor 15 are compared by a comparator 100. In response to the output of the comparator 100 a speed controller 22 applies a speed control signal to an absolute value convertor 23 and a function generator 27, the former forming a current instruction signal and the latter a slip frequency instruction signal.
The current instruction signal is compared with a current signal detected by a current transformer 101 by means of a comparator 102, and the result of comparison is applied to a current controller 24 to form a current control signal which is applied to a phase controller 25 for controlling the ignition of the thyristors of the rectifier.
The slip frequency instruction signal is compared with the output of the speed detector 16 by means of a comparator 103 and the result of comparison is applied to an oscillator 29 to form a pulse signal. The frequency of this pulse signal is divided by a frequency divider 30 and the output thereof is applied to a pulse amplifier 32 for controlling the ignition of the thyristors of the inverter 14.
With the control system described above, the slip frequency and the secondary current of the induction motor 15 are controlled in a predetermined correlated manner so as to maintain the magnetic flux of the motor at a substantially constant valve.
FIGS. 2a and 2b are vector diagrams showing the powering and regeneration mode operations of the motor, in which .PHI. represents the magnetic flux, E.sub.1 the primary voltage, I.sub.1 the primary current, E.sub.2 the secondary induced voltage, I.sub.2 the secondary current, I.sub.0 the exciting current, .phi. the power factor angle, and .theta. the angle between the magnetic flux .PHI. and the primary current I.sub.1 and termed a current phase angle.
When the speed of an alternating current motor is controlled by a current control type frequency converter described above, the current phase of the primary current I.sub.1 is determined by the gate signal applied to the thyristors as is well known in the art. Since the current phase is fixed, under a transient state in which the operation of the motor is changed from the powering mode to the regeneration mode or vice versa, the phases of E.sub.1, E.sub.2 and .PHI. vary to establish the desired phase relationship for the new operation state.
The speed at which the phases of the induced voltage and the magnetic flux vary is determined by the secondary time constant EQU T.sub.2 =(L.sub.2 +L.sub.m)/R.sub.2
where
L.sub.2 : secondary circuit inductance as viewed from primary side PA1 L.sub.m : excitation mutual inductance of the motor PA1 R.sub.2 : secondary circuit resistance as viewed from primary side.
Generally, the secondary time constant T.sub.2 is of the order of several hundred milliseconds so that even when the polarity of the output of the speed controller is reversed and an instruction for changing the operation mode from powering to regeneration or vice versa is produced, desired torque would not be produced until the phase of flux .PHI. stabilizes at a desired phase angle. For this reason, under these transient states, even when the primary current is increased the torque would not increase correspondingly. Furthermore, when the load varies during the powering or regeneration mode operation the phase of the flux .PHI. should be varied in order that the power factor angle .phi. becomes a predetermined value so that the torque would not increase corresponding to the increase in the current under transient state. In short, with the prior art control system, since the current phase is fixed it is impossible to obtain desired torque under transient state and hence quick response speed control.